1. Field of the Invention
This invention relates to a scanning optical system for eliminating any irregularity of the pitch of the scanning lines and having a relatively bright F number (i.e. small) in the main scanning direction.
2. Description of the Prior Art
In conventional light beam scanning devices using a deflecting-reflection surface, such as, a rotating polygon mirror, there have been proposed various kinds of scanning optical systems in which the advancing direction of the light beam deflectingly scanned by the deflecting-reflection surface is deviated, due to a shift of the deflecting-reflection surface, relative to a surface perpendicular to the deflection surface, which is formed by rotating the line normal to the deflecting-reflection surface. Consequently, there occurs an irregularity of the pitch of the scanning lines.
For example, in Japanese patent publication No. 28666/1977, which corresponds to U.S. Pat. No. 3,750,189, the optical system between the deflecting device and the medium to be scanned consists of beam shaping means and second beam focussing means so as to collimate the light flux reflected by the deflection mirror by said beam shaping means. When such a collimating function is provided a restrictive condition is required for the shape of the beam shaping means to keep constant the focussing property on the surface to be scanned and the speed of focussing, the strain is produced and the freedom for improving the performance characteristics is decreased and it is difficult to obtain an excellent performance characteristics unless the number of lenses constituting the second beam focussing system is increased.
In the Japanese patent laid-open application No. 93720/1975, which corresponds to U.S. Pat. No. 3,946,150, a cylindrical lens is disposed between the lens having strain property of realizing equal speed scanning and the medium to be scanned. With this arrangement, it is not possible to obtain a good image unless the cylindrical lens is located near the medium to be scanned. When the cylindrical lens is located close to the medium to be scanned the scanning width becomes long so that it is difficult to realize a compact structure.
In the Japanese patent laid-open application No. 36622/1981, corresponding to U.S. Pat. No. 4,379,612, an optical system consisting of a spherical single lens and a single lens having a toric surface is disposed between the deflector and the medium to be scanned to give a strain property and the function to compensate for the shift of the deflecting-reflection surface. In this arrangement, although a compact structure is obtained, two single lenses are provided in the main scanning direction, the freedom is less and it is not possible to reduce the spreading of the stop in the main scanning direction.
In recent laser beam scanning optical systems, it has been required to provide gradation within 1 dot by dividing 1 dot into two or three parts only in the main scanning direction. For dividing 1 dot into two or three parts in the main scanning direction, the pulse width of the laser is changed. Thus, it is necessary, within a small pulse width of less than 1 dot, that the width of the beam spot in the main scanning direction is made equal to the length on the surface to be scanned corresponding to said pulse width.